Remove Filesystem Imports

Attention: This is a breaking change! (Search below for the term API-CHange).

This MR changes the way miniflask imports modules.
Previously, miniflask would overwrite the python import-routine to allow module repositories in the whole filesystem.
This however, leads to a variety of problems, for instance harder to track errors in testing environments, instantiations of objects for relative imports compared to instantiations given through miniflask-routines, etc.

The solution to all this is to let miniflask determine the import paths that python would use for all module imports and use those for importing.
With this in place, renaming module-base directories internally, or, allowing filesystem-wide imports is not possible anymore.

Nevertheless I strongly believe that making miniflask mimic python's behavior for module naming is more natural and surely more future-proof with regards to understanding the internal behavior as a user and also for debugging for later developments.

*Note: Merging this PR closes #115 and closes #97.

Example

Consider the following directory-structure:

myproject (main project directory)
├── main.py
├── __init__.py
├── subpackage
│   ├── __init__.py
│   ├── usingminiflask.py (first file that uses miniflask)
│   ├── modules
│   │   ├── __init__.py
│   │   ├── module1/ (contents omitted)
│   │   ├── otherdir
│   │   │   ├── __init__.py
│   │   │   ├── module2/ (contents omitted)
....

• Note that usingminiflask.py sits itself in a python package named subpackage. This is the actual file that calls mf.init in a way.
• Considering usingminiflask.py, the accessible modules are called modules.module1 and modules.otherdir.module2.
• Previously, the have been included by overwriting the python-internal importlib-spec objects as the packages modules.module1 and modules.otherdir.module2.
• With this PR in place, their internal importnames are myproject.subpackage.modules.module1 and myproject.subpackage.modules.otherdir.module2, as myproject.subpackage is the actual python-prefix for all modules.
• I am free to discuss whether these prefixes (myproject.subpackage) should be also added to the miniflask-ids, however, the current design decision is not to do that. In theory, such a change should be doable without breaking to much, however I am not yet sure to be honest. In theory some users might be depending on absolute paths to work. If for instance those do not rely on state.all, everything should be fine, though.

Things done in this MR

The core commit for this MR is probably 83f69641, where the true python import name of each module specification is determined and used to import a module.

There are other things that needed to be done to make this a complete function. Here in more detail:

• API-Change: As filesystem-wide imports and renamings are not possible anymore, this MR removes the possibility to give mf.init a dict for initialization.
• API-Change: We have allowed module names with a .. This is not possible anymore due to the requirement of being also python-import-compatible.
• Refactored the module_spec["importname"] to be exactly the same as python's import-name for the respective modules, also for miniflask's pre-shipped modules.
• Fixed a minor bug, where base-directory-modules would end with a ..
• To make the tests work with pytest on every level, all tests needs to be a python-packages themselves.
  I used the opportunity to normalize the folder-structure a bit.
• Added some tests to show how to choose only some modules from a module repository.

Check all before creating this PR:

• Documentation adapted
• unit tests adapted / created